The Role of Protons and Formation Cu(NH3)2+ During Ammonia-Assisted Solid-State Ion Exchange of Copper(I) Oxide into Zeolites
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Solid-state ion exchange in mixtures of copper oxides and zeolites can occur at temperatures as low as 200–250 °C in the presence of ammonia (NH3-SSIE), thus providing a low-temperature method to activate zeolites for selective catalytic reduction nitrogen oxide (NH3-SCR). The ammonia induced solid-state ion exchange process is studied in more detail, by monitoring the development of NH3-SCR activity with duration of NH3-SSIE, formation of a mobile linear [Cu(NH3)2]+ complex with X-ray spectroscopy, and the transfer of Cu to the zeolite by XRD, and evaporation of copper(I)-oxide by TGA. We find that the linear [Cu(NH3)2]+ complex is formed in mixtures of copper-oxide and *BEA and CHA zeolites upon exposure to ammonia. Increasing the temperature for NH3-SSIE to well above 300 °C leads to a less efficient Cu-transfer. This indicates that the [Cu(NH3)2]+ complex is crucial for the NH3-SSIE process. The non-monotonous development of NOx conversion and N2O yield with duration of NH3-SSIE is probably due to an initial enrichment of Cu in the outer shell of the zeolite crystals. We also show that the presence of H+ or NH4+-ions in the zeolite are necessary for the NH3-SSIE, and that the transfer of Cu from the Cu-oxides to the zeolites most likely occurs via a surface diffusion process.
KeywordsZeolites Solid-state ion exchange Selective catalytic reduction Cuprous oxide